1. Field of the Invention
The present invention relates to an optical device, and more particularly to a wavelength division multiplexer.
2. Description of the Related Art
An arrayed waveguides grating (hereinafter, referred to as an AWG) can be used as a wavelength division multiplexer/demultiplexer in an optical transport network. If the AWG has a box-like flat transmission band characteristic, it is called a flat-top AWG. Flat-top AWGs can increase the drift tolerance of a wavelength of a light source. When the flat-top AWG is continuously used, an entire transmission bandwidth can also be maintained. FIG. 1 is a diagram showing the construction of a typical flat-top AWG FIG. 2 is an enlarged diagram of an input part in FIG. 1. FIG. 3 is an enlarged diagram of an output part in FIG. 1.
As shown in FIG. 1, the AWG 100 includes an input waveguide 110, a mode converter 120, a first slab waveguide 130, a plurality of channel waveguides 140, a second slab waveguide 150, a plurality of output waveguides 160.
Referring to FIG. 2, the mode converter 120 expands the width of a mode 170 of an optical signal input from the input waveguide 110. As a result, fundamental mode 172 and a secondary mode 174 are generated. The mode converter 120 also adjusts the energy distribution between the modes 172 and 174. A mode 176 of an optical signal incident into an end surface 132 of the first slab waveguide 130 is converted from a Gaussian function shape to a flat-top shape. The mode 176 includes a flat zone having a width of ΔF5.
When the flat-top AWG 100 is used in two bands (e.g., O-band and C-band) which have a large wavelength difference, a difference between transmission bandwidths occurs. Referring to FIG. 3, when a wavelength shifts from a transmission central wavelength λc by Δλ, an image 178 (has a similar shape as that of the mode 176 of the optical signal incident into the first slab waveguide 130) formed on an end surface 152 of the second slab waveguide 150 has a spatial positional variation Δx defined by equation 1.
                              Δ          ⁢                                          ⁢          x                =                                            ⅆ              x                                      ⅆ              λ                                ×          Δ          ⁢                                          ⁢          λ                                    Equation        ⁢                                  ⁢        1            
In equation 1,
      ⅆ    x        ⅆ    λ  represents positional variation (or a distance by which a focus of an image plane moves) of an image with respect to a unit wavelength variation for the transmission central wavelength λc, and is defined by equation 2.
                                          ⅆ            x                                ⅆ            λ                          =                                            N              c                        ⁢            f            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            L                                              n              s                        ⁢            d            ⁢                                                  ⁢                          λ              c                                                          Equation        ⁢                                  ⁢        2            
In equation 2,
                    N        c            ⁢                          ⁢              (              =                  n        c            -              λ        ⁢                                  ⁢                              ⅆ                          n              c                                            ⅆ            λ                                ,where nc represents an effective refractive index of the channel waveguides 140) represents a group refractive index of the channel waveguides 140, f represents lengths (or focal lengths of the channel waveguides 140) of the first and the second slab waveguide 130 and 150, ΔL represents difference of lengths between adjacent channel waveguides 140, ns represents effective refractive indices of the first and the second slab waveguide 130 and 150, and d represents intervals between adjacent channel waveguides 140.
When a width of a flat zone of the image 178 formed on the end surface 152 of the second slab waveguide 150 is ΔF6(≈ΔF5), the flat transmission bandwidth Δf is defined by equation 3.
                              Δ          ⁢                                          ⁢          f                =                              Δ            ⁢                                                  ⁢                                          F                6                            /                                                ⅆ                  x                                                  ⅆ                  λ                                                              =                                                                      n                  s                                ⁢                d                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                                  F                  6                                                                              N                  c                                ⁢                f                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                L                                      ⁢                          λ              c                                                          Equation        ⁢                                  ⁢        3            
Accordingly, when the flat-top AWG 100 is used in two bands, Δf has difference. Hereinafter, the O-band and C-band will be described as an example. Difference of
      ⅆ    x        ⅆ    λ  in two bands is about 17%, but ΔF6 shows nearly no difference since the two bands use the same mode converter 120. In result, the transmission bandwidths have difference of about 17%.
As describe above, it is difficult to use the typical flat-top AWG in two bands due to the difference between transmission bandwidths.